Use of transponder on the ground

[Zoyberg]

Use of transponder on the ground
I asked this question in "tech log" and no one was able to answer the question where I can find out if I should select xpdr on the ground (where it does not say in my aerodrome booklet). Can anyone here help please?

It does not give this as a requirement in our SOPs.

It does mention in many aerodrome booklets that transponder use is required during taxy.

There are many large airfields I visit in Europe and the Middle East and Asia where I am told by the trainers it is required (even though this is not mentioned in the booklets or the Jeps area supplement).

Anyone know where this is written please?

[Gonzo]

UK AIP

London Heathrow

EGLL AD 2.20 Section 2 a

viii Heathrow Airport is equipped with an advanced surface movement radar utilising Mode-S.
1 Aircraft operators intending to use London Heathrow Airport should ensure that Mode S transponders are able to operate when the aircraft is on the ground
2 Flight crew should:
Select XPNDR or the equivalent according to specific installation, AUTO if available, not OFF or STDBY, and the assigned Mode A code.
From the request for push back or taxi whichever is earlier;
After landing, continuously until the aircraft is fully parked on stand
After parking the Mode A code 2000 must be set before selecting OFF or STDBY.
3 Flight crew of aircraft equipped with Mode S having an aircraft identification feature should also set the aircraft identification. This setting is the aircraft identification specified in Item 7 of the ICAO ATC Flight Plan. The aircraft identification should be entered from the request for push back or taxi, whichever is earlier, through the FMS or the Transponder Control Panel.

[adverse-bump]

Select XPNDR or the equivalent according to specific installation, AUTO if available, not OFF or STDBY, and the assigned Mode A code.

There alot of confusion at my company as to what this means. The way I read it, whilst taxiing the transponder should be in the same position it is during flight. But some crews get confused as it mentions Mode A, and have started turning txponder to Mode A only.

Surely this will mess up everyone's tcas as they fly overhead? As if there is no mode C turned on it will set off a TA even if 9900 ft above.

[Radarspod]

This matter does cause some confusion, I agree.

Regarding the setting transponder, assuming you are operating some form of air transport category aircraft, your transponder fit is likely to have a strut switch or some other means of determining whether your aircraft is on the ground or airborne. This state determination is used to control replies to Mode A/ Mode C (ATCRBS) / Mode S All-call interrogations, such that when on the ground you are not replying to these interrogations from airfield radars. This will be the case when operating in XPDR or AUTO depending on your aircraft configuration. This also determines whether you are ellicting airborne or ground squitters.

I believe that the usual SOP for transponder was to be in STNDBY or OFF then set to XPDR or AUTO or TA/RA just before you line up (not a pilot so don't quote me on that bit). However, if you're at EGLL say, if you were in STNDBY or OFF, your transponder would not be squittering and you would not be appearing on ASMGCS displays for GM. By setting your Mode A at the gate and enabling the XPNDR, you appear to ASMGCS and GM can track you round the airport. You also get your squawk validated at the gate.

Regarding TCAS, the replies elicited from Mode S equipped aircraft on the ground by the TCAS interrogations of overflying aircraft contain an indication of the airborne / ground state of the aircraft. The TCAS on the overflying aircraft therefore does not track the aircraft on the ground as it knows it is on the ground, hence should not generate TA or RA. As such I don't think in this situation it is necessary to select Mode A only. In fact, in today's environment, unless your altitude encoder went screwy, there is no reason to ever select Mode A only.

The only reason you would NOT select XPDR on the ground is if you aircraft does not have an automatic means of working out if it is on the ground or not, else you will be replying to the airfield or nearby SSR radar.

However, some transponder manufacturers (Garmin and Trig spring to mind) market a GA transponder that has an additional GROUND setting - this allows the aircraft to provide squitters on the ground to enable detection by ASMGCS yet at the same time not reply to Mode A / Mode C / Mode S All-calls from the airport radar.

All of the above is general and will depend on aircraft fit.

RS